Well, with the proper coating iron could be used. The problem is corrosion. If you mount a microstrip circuit to the floor of an iron housing, and you have coaxial connectors going out of the side, you need the iron housing to make excellent contact to the bottom of the microstrip in the vicinity of the connector, and to the underside of the connector too. Otherwise, you would not have a good ground connection between the connector and the microstrip backside.
Something like nickle plate over iron/steel would work. Steel is preferrable to iron, since you will want to drill and tap small holes.